TS 320 

.S5 
Copy 1 



DEPARTMENT OF COMMERCE 



Scientific Papers 

OF THE 

Bureau of Standards 

S. W. STRATTON. Director 

No. 395 

relation of the fflGH-TEMPERATURE TREAT- 
MENT OF HIGH-SPEED STEEL TO SECONDARY 
HARDENING AND RED HARDNESS 



HOWARD SCOTT, Assistant Physicist 
Bureau of Standards 



SEPTEMBER 16, 1920 



2 D - ^ fc "1 "^ "^ 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents. Government Printing OflBce 
Washington, D. C. 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1920 






RELATION OF THE HIGH-TEMPERATURE TREAT- 
MENT OF HIGH-SPEED STEEL TO SECONDARY 
HARDENING AND RED HARDNESS 



By Howard Scott 



CONTENTS 



Page 



I . Introduction 521 

II. Physical properties and structure of high-speed steel S^^ 

1 . Effect of quenching temperature 524 

2. Effect of tempering temperature 527 

III. Signilicance of the physical characteristics of high-speed steel 535 

IV. Summary 536 

I. INTRODUCTION 

The metallography of high-speed tool steel presents certain 
anomalies, the explanation of which is not clear from the usual 
conceptions of the mechanism of hardening in simple carbon 
steels, as, for example, the familiar polyhedral structure of prop- 
erly quenched high-speed steel, which is often called austenite, 
although its physical properties are largely those of martensite. 
The explanation of such anomalies and a better understanding of 
the fimdamental nature of high-speed steel is becoming more and 
more important as the peak of its development is being reached. 
It is even probable that future improvements will be largely in 
the technique of treatment and the adjustment of compositions 
to meet special requirements, in which cases fundamentals are of 
highest importance. 

From this angle the problem of developing high-speed steel is 
one of constitution and not composition. In spite of a wide 
variety of compositions, the general characteristics are very 
similar. The most effective method of attack is, therefore, 
through the study of a number of the physical properties of a 
high-speed steel and their correlation with the corresponding 
characteristics of simple carbon tool steel, the recognized reference 
standard. The relations for carbon steels between heat treat- 
ment, microstructiu^e, and physical properties have been rather 
thoroughly studied and are summarized in another paper.' 

* Scott and Movius. see forthcoming paper. Thermal and Physical Changes Accompanying the Heating 
of Hardened Carbon Steels. 

S2I 



^0 



^^m 



522 Scientific Papers of the Bureau of Standards [voi. i6 

To make the proposed correlation, the microstructiire, hard- 
ness, density, magnetic properties, and thermal characteristics of 
a typical high-speed steel as affected by various treatments were 
studied. The variety of properties investigated necessitated the 
cooperation of several other laboratories of the Bureau of Stand- 
ards; H. S. Rawdon prepared the micrographs, E. L. Peffer made 
the density determinations, R. L. Sanford the magnetic tests, 
F. H. Tucker the chemical analyses. Miss H. G. Movius prepared 
the thermal curves, and several assistants aided in the work. 

The previously published researches related to the present one 
are those of Edwards and Kikkawa,^ Carpenter,' Yatsevich,* 
Mathews,^ and Andrew and Green." 

Edwards and Kikkawa determined the effect of tempering 
temperature on the Brinell hardness of two series of chrome- 
tungsten steels of constant carbon content, chromium being vari- 
able in one series and tungsten in the other. A constant harden- 
ing temperature of 1350° C was used with one exception, and the 
density changes of one representative composition were deter- 
mined. This work represents the most important contribution to 
date to the study of the physical properties of high-speed steels. 

Carpenter investigated the effect of quenching and tempering 
temperatures on the structure and etching time of some high- 
speed steels. 

Carpenter, Yatsevich, and Andrew and Green studied the critical 
ranges of high-speed steels as affected by maximum temperature 
and rate of cooling. 

Mathews summarized his wide experience with cutting and 
physical tests of high-speed steel and reviewed developments 
since the classic experiments of Taylor and White. 

These papers represent work on high-speed steel of a wide variety 
of compositions and treatments, so that it is very difficult to 
establish any relation between the various properties studied. 
The present work is, therefore, confined to one representative 
type of modern high-speed steel. 

II. PHYSICAL PROPERTIES AND STRUCTURE OF HIGH- 
SPEED STEEL 

For the purpose of this investigation tests were made on a 
standard type of high-speed steel, chosen because it shows sec- 

3 Edwards and Kikkawa. Jour. Iron and Steel Inst., 92, p. 6; 1915. 

2 Carpenter, Jour. Iron and Steel Inst.. 07. p. 433, 1905; 71, p. 377, 1906. 

* Yatsevich, Rev. deMet., 15. p. 65; 191S. 

s Mathews. Proc. A. S. T. M., 19, p. 141; 1919. 

^ Andrew and Green, Jour. Iron and Steel Inst.. 99. p. 305; 1919. 



Scientific Papers of the Bureau of StDndard^, V^-l )f< 




Fig. I. — Hanlcnnnj cracks in high-speed siccl 



Specimens of steel B. i by ^s by 3 inches, were harden 
1030" C; surface of fracture parallel td i by 3 inches surft 
perpendicular to i by 3 inches surface; c, quenched in oi 
45^^ to I by 3 inches surface 



as follows: (I, Quenched in oiUrom 
; /», cooled in air from lo^o" C; cracks 
ona 1050° C; fracture paralleland at 



Heat Treatment of High-Speed Steel 



523 



ondary hardening definitely. Unfortunately, sufficient material 
of one composition was not available, so that another composi- 
tion was used also, this being purchased to duplicate the first as 
nearly as possible. As seen from the chemical composition given 
in Table i, the two steels are very similar except in carbon con- 
tent, which is low in the B steel for the brand used. As pointed 
out later, the lower carbon content, apparently, is responsible for 
a poorer steel. 

TABLE 1.— Results of Chemical Analyses of High-Speed Steels 



No. 


C 


Mn 


SI 


W 


Cr 


V 


P 


s 


A 


Per cent 

0.77 
.65 


Per cent 
0.25 
.31 


Per cent 

0.47 
.17 


Per cent 

17.8 
17.6 


Per cent 
3.5 
3.4 


Per cent 

0.74 
.73 


Per cent 

0.020 
.004 


Per cent 


B . .. 









For hardening, the specimens were placed in an electrically 
heated alundum tube furnace in which charcoal or illuminating 
gas was biuned to prevent excessive oxidation. The specimens 
were slowly brought up to temperature and held there 15 minutes 
before quenching. The quenching medium was a light mineral 
oil. Tempering consisted of heating for 15 minutes in an oil 
bath for temperatures up to 250° C, in a nitrate bath up to 
600° C, and in a chloride bath for higher temperatures. The 
use of these baths assured a uniform temperature quickly reached. 
All high-temperature measurements were made with platinum 
thermocouples. 

The specimens were cut from a i-inch square bar (steel A), 
and a I by 5/^ inch bar (steel B), both factory annealed. The 
magnetic test specimens were of i cm square cross section, the 
hardness specimens were of i by X inch (steel A), and i by ^ 
inch (steel B) section, ajid the density specimens about i by i 
by ^2 inch (Fig. 7) and about one-half inch face cubes (Fig. 5). 
Specimens for micrographs were taken from the ends of the 
hardened magnetic test pieces. 

In hardening, both steels usually cracked when quenched from 
the region of 1050° C. The cracks followed the contoiu" of the 
specimens, thus indicating that they were characteristic of the 
steel and not due to inclusions. The cracking was particularly 
severe in the case of the low-carbon steel B, photographs of 
typical samples of which are shown in Fig. i (a, b, and c). This 
steel cracked also on cooling in air from 1050° C. It was, how- 



524 



Scientific Papers of the Bureau of Standards 



ever, possible to obtain specimens on which the physical measure- 
ments could be satisfactorily made in spite of the cracks. 

The density measiu-ements were made by the usual method of 
weighing in air and in water, the specimens being dipped in 
alcohol prior to immersion in water to insure the absence of 
bubbles. The magnetic measurements were made in a long 
solenoid, corrections being made by the use of shearing curves. 
Other tests were made by the usual standard methods. 

1. EFFECT OF QUENCHING TEMPERATURE 

It has long been recognized that raising the quenching 
temperature increases the cutting efficiency of a high-speed steel 





1 1 1 1 ■■ 
ooA Quenched /n o// 


/4000 


• ■ 


A Same sjbec/'mens after 

d/fop/n^ /'n //i^ciid a/r 
. 1 1 1 


/oooo 


















\l^/r} 






6000 














SiZfe 


"-^ 


£r 




^ 


f 


bOOO 








^^J;'?*^ 


M 




'/^OOO 








^K; 


^ 








^^ 


^^ 


X 






2000 




^ 






\\ 




y 




/-/- /SO 




\ 





1 




\ 



//a 
80 



eoo 900 woo //OO /^OO /JOO'C 
Quenching temperature 

Fig. 4. — Relation of quenching temperature and subsequent 
treatment in liquid air to maximum induction, residual 
induction, and coercive force of steel B 

tool, so that the highest temperature short of fusion is the best. 
Observations were therefore made to determine the effect of 
quenching temperature on the properties under consideration to 
obtain evidence as to the nature of the constitutional changes. 



Scientific Paper-, of t 



pWW^'' 



of standards. Vol. 16. 




a- /7s rece/Vec:/ 



/^- 900 "C 




.■.•'1-*' ■. 



c/'/r;,f^"r 



e - //<?5" °C f- /^OO "C 



j^:r^ 



Fig. 2. — Miiiosliucturc of specimens of steel A quenched from tcnipcraluto noted, 
y fiuo. Etched icith 2 per cent alcoholic HNO3 

a. Annealed. ((0.900° C. (i:).975° C, W), 1050° C; e. 1125° C, /, 1200° C 



Scientific Papers of t 




r"lG. 5. — Microsinicture of specimens of steel A quenched from tcmperaiuia noted. 
Xjoo. Etched with 2 per cent alcoholic HNO:^ 

g, i22c° C; k, 1290° C, interior; i, 1290° C, structure near surface 



SwU] 



Heat Treatment of High-Speed Steel 



525 



The microstructure of steel A as quenched from several tempera- 
tures is shown in the micrographs of Figs. 2 and 3, the magnetic 
properties in Fig. 4 (lower curves of shaded areas), and the 
density in Fig. 5. The effect of quenching temperature on the 
Brinell and scleroscope hardness (recording instrument) of steel 
B is also shown in Fig. 5. 
roOr 



^600 



■^30 



,eo 



8.b5 




^d.i 



Sfee/ 3 -Srine// hc?r dries s 
>Sfee/ £ - Sc/ero3co/De hardness 
■Sfee/ /? -Densify 



800 900 /OOO 1/00 /^OO /JOO'C' 

Quench/n^ temperature 

Fig. 5. — Relation of quenching temperature to Brinell and 
scleroscope hardness of steel B and to density of steel A 

These data permit of a classification of the quenched specimens 
into two groups according to the nature of their physical character- 
istics. The properties of the first group, quenched from tempera- 
tures up to about 1 100° C, vary in a manner distinct from those 
of the second group, quenched from above that temperature. 
Besides the change in slope of the curves, the physical properites 
of the two groups are affected differently on cooling below ordi- 
nary temperatures. Thus the magnetic properties of the speci- 
mens quenched from the high temperatures are markedly in- 
creased (Fig. 4, upper curve of shaded area) by immersion in 
liquid air, while the specimens quenched from the low temperatures 
remain practically unchanged. This is indicative of a con- 
stitutional difference, other than a continuously changing one, 
between the specimens of the two groups and has an important 
bearing on the anomalies of high-speed steel. 



526 Scientific Papers of the Bureau of Standards [vu. t6 

To distinguish between these two groups, specimens quenched 
from the lower temperature range will be referred to as given the 
low-temperature treatment, and those from the upoer range as 
given the high-temperature treatment. 

The microstructure of the samples given a low-temperature 
quench is obscured by the excessive amoimt of free carbide im- 
bedded in the apparently structureless matrix. The micro- 
structure of the specimens of this series will be called martensite 
from their physical characteristics, but it must be recognized that 
this may be a misuse of the term, depending, of course, on its 
definition. The specimens quenched from the high-temperature 
range show well-defined grain boundaries in a structureless matrix 
containing little free carbide. This structure is typical of properly 
quenched high-speed steel and will be called polyhedral. 

The polyhedral structure, smaller volume change, constancy 
of hardness, and more rapid loss in magnetization with quenching 
temperature in this range are all suggestive of austenitization, 
though the polyhedral structure is not necessarily proof of it. It 
is seen by extrapolation of the magnetic properties that zero 
magnetization, and hence complete austenitization, would be 
attained for a quenching of about 1450° C if this temperature 
could be reached without fusion. That partial austenitization 
has occurred on quenching from the high-temperature range is 
shown, however, by the changes in physical properties on cool- 
ing below ordinary temperatures. This treatment, if carried 
to a low enough temperature, completes the A3 transformation 
with a corresponding change in physical properties, direct evi- 
dence of previous austenitization. The effect on the magnetic 
properties of immersion in liquid air is shown in Fig. 4. The 
volume also increases on cooling below ordinary temperatures, a 
drop in density of 0.059 g/cm^ being observed when a specimen of 
steel A, quenched from 1300° C, was cooled to —45° C. It must, 
therefore, be concluded that the specimens quenched from the 
high-temperature range are constitutionally diflferent from those 
given the low-temperature treatment in that in the former case 
the steel is partially austenitic, but is not in the latter. 

This characteristic of high-speed steel may appear peculiar to 
it, but on reference to the work of Maurer' one will find that more 
or less partial austenitization is common to simple high-carbon 
steel quenched from a high temperature, the degree depending, 
of com-se, on the carbon content and the temperature. This 

' Maurer. Rev. deMet.. 5, p. 711; 1908. 



Heat Treatment of High-Speed Steel 



527 



phenomenon is revealed by the change in density and in other 
physical properties on immersion oi the steel in liquid air. 

A further analogy between carbon and high-speed steel may be 
seen by comparison of the effect of the quenching temperature 
on the magnetic properties of high-speed steel (Fig. 4), with the 
effect of the same variable on those of a carbon steel (Fig. 6). 




rCO ^ rSO dOO 3S0 900 °c 

Fig. 6. — Relation of quenching lempcrature to 
coercive force, maximum induction, and residual 
reduction of carbo n tool steel. (Gebert) 

The data for the carbon steel were taken from Gebert.* The 
similarity of these two figures is striking Avhen the lack of any 
similarity in microstructure is considered. 

2. EFFECT OF TEMPERING TEMPERATURE 

The most interesting feature of the tempering of high-speed 
steel is the so-called "secondary hardening," which is revealed 
as an increase in hardness over the original (or a previous minimum) 
of certain high-speed steels given the high-temperature treatment 
and tempered in the neighborhood of 600° C. This, at first sight 
and in view of its absence in the usual carbon tool steels, is often 
considered a mysterious phenomenon. However, when the 
original condition of partial austenitization resulting from the 
high-temperatiu'e treatment is considered, the phenomenon 
appears quite natural. 



« Gebert. Proc. A. S. T. M., 19, Part II; p. 117; 1919. 



528 



Scientific Papers of the Bureau of Standards 



IV0I.T6 



The effect of increasing the quenching temperature is to increase 
the amount of dissolved carbide which lowers Ar" (Arg^, of 
martensitic steels, which is long and continuous for high-alloy 
contents) progressively, until, for a quenching temperature of 
about 1100° C, its end reaches room temperature. Any further 
increment of the quenching temperature — that is, quenching from 
the high-temperature treatment range— will cause the end of Ar" 
to fall below ordinary temperatures, with the result of partial 
austenitization already noted. This phenomenon is analogous 
to the lowering of Arj in iron-nickel alloys by increasing the nickel 
content, the essential difference being that in the preceding case 
the composition of the matrix can be changed by tempering, but 
it can not be so changed in the latter. 

From the foregoing analysis it is evident that on tempering the 
partially austenitic, and consequently somewhat softened, steel, the 
dissolved carbide of the matrix will be gradually precipitated until 
a stage is reached at which Ar" is no longer stable for the then 



f^ 


1 1 




d60 


C.77. w /r.e, CrJ.s, I' 
Quenched in o/7 
from : 
900 "C 


^.ru 


8bZ 


• /ni,o °c 




y^ \ 




i 1^20 °C 


J 


\ 




* /300 °C 


/jooyi 


\ ^\ 




,^^ 




// 


/^zo\ \ 


&&V 


^■^^^ 




// 


\v 




X^ 




7^ 


-.V 




VN 




// 








1 






1 









o zoo ^00 &00 eoo'c 
Temjber/ny temperature 

Fig. 7. — Change in density of steel A with tem- 
pering temperature 

existing temperature and composition of the matrix. Ar" will 
then complete itself as in the case of the austenitic carbon steel 
discussed in another paper.^ The consummation of Ar" implies 
martensitization, and consequently an increase in hardness, which 
the physical property and microstructiue data verify. 

'See footnote i. 



Scott] 



Heat Treatment of High-Speed Steel 



529 




200 ^00 MO 

Tempering temperature 

Fig. 8. — Change in sckroscope hardness of s 
■with tempering temperature 




ZOO ^00 i>00 800X 

Tempering temperature 

Fig. g.—Cliange in coercive force of steel A with 
temperimg temperature 



530 



Scientific Papers of the Buremi of Standards 



The effect of tempering temperature on the physical properties 
of steel A quenched from several temperatures is shown in Figs. 
7 (density), 8 (scleroscope hardness), 9 (coercive force), 10 (maxi- 
mum induction) , and 1 1 (residual induction) . The reciprocals of 
the density values have been plotted to show more clearly their 




4/(?^^ 



^am 



jSOO ^00 iOO SOO'C 

Tempering tem/jerature 

Fig. 10. — Change in maximtnn induction of steel A 
with tempering temperature 

relation to the hardness values. In Fig. 12 are given Brinell 
and scleroscope curves for steel B. 

The hardness curves (Figs. 8 and 12) show the phenomenon of 
secondary hardening only when the steel was quenched from the 
high-temperature range, above 1100° C, thereby confirming its 
relation to partial austenitization noted only in specimens 
quenched in the same temperature range. The curves represent- 
ing the results of the low-temperature treatment are similar to 
those of carbon steels with the exception that tempering occurs 
at a much higher temperature, thus shortening the temperature 
range over which troostite is stable, with the result that the 
change from martensite to sorbite, or complete softening, is quite 



ScotI] 



Heat Treatment of High-Speed Steel 



531 



abrupt. The density change is very small, indicating a slight 
solubility of the carbide in the low-temperature treatment range. 




o zoo ^00 (>oo aoo'c 

Jemper/ni^ temjberatcjre 

Fig. II. — Change in residual induction of steel A 
with tempering temperature 

Of chief interest is the tempering of specimens given the high- 
temperature — that is, the most efficient — hardening treatment. 
The density curves (Fig. 7) show very markedly a peak between 
600 and 700° C, which is parallel to the hardness curves and cor- 
responds to a large increase in volume. This is direct evidence of 
martensitization, and the magnetic properties offer further con- 
firmation. The increase in maximum and residual induction 
(Figs. 10 and 11), normally indicative of softening, accompanies 
the rise in hardness in the secondary range. 

Secondary hardening, as defined here, has been looked upon as 
a feature peculiar to high-speed steel. The work of Maurer, how- 
ever, shows that hypereutectoid carbon steels quenched from the 
neighborhood of Acem exhibit the same phenomenon, as revealed 
by density measurements, to a degree dependent on the amount 
of carbide retained in solution. The only difference between the 
high-carbon and the high-speed steel is that the peak comes at a 



532 



Scientific Papers of the Bureau of Standards 



much lower temperature in the former case, and the tempering 
below the peak is naturally more evident. The increase in inten- 
sity of the peak with an increasing degree of austenitization is 
evidence of a general relation between secondary hardening and 
partial austenitization. 




^00 '/OO {>00 800 'C 

Tempering tem/jerature 

Fig. 12. — Change in Brinell atid scleroscope hard- 
ness of steel B with tempering temperature 

The conclusions arrived at from an examination of the physical 
changes on tempering as related to the phenomenon of secondary 
hardening should be capable of verification by observation of the 
accompanying microstructural changes. Micrographs of speci- 
mens of steel A quenched from 900, 1050, 1200, and 1290° C are 
given in Figs. 13, 14, 15, and 16, respectively, as tempered at 
several temperatures. The first two figiu-es show the structure 
resulting from a low-temperature treatment and the second two 
from a high one. 




Fig. i3.--,l/u-)0i/n(f/Hu' of sled A 
quciuiud in oil from goo" C and 
tempered as noted. X500. Etched 
with 2 per cent alcoholic HNU^ 
a. 300° C; h. 400° C; ,. ftoo° C. rf, 700° C 



Vu;. 14- Micioslrucluie of sldl A 
quenchid in oil jtom lujjo" (' and 
tempered as noted. X500. Etched 
with 2 per cent alcoholic HNOi 
a, 300° C; h, 4co^ C; c, 600" C; J, 700" C 



Scientific Papor^ r.f ; 



3U of Standird', V: 1 1« 







{ 3 i».-» 



-■ >.■ - ■-•■'• .■W 

(^7- Quenc/he<:/ /n /6 - (2uer?c/7ea' /n 

0/7 from /^^O °C, o/V frorr? /300 °C 
fe/ryferec/ a/ SOO °C. anc^ c//jbhec^ in 

Fig. i-;—MiiiOiliiiiluu ofiUilA. > joo. Elch,d u ill: 2 per cent alcoholic HXO^ 



Scott] Heat Treatment of High-Speed Steel 533 

Micrographs a and b of Figs. 13 and 14 (low-temperature treat- 
ment) show an immature martensitic pattern resulting from 
tempering at 300 and 400° C. At 600° C the structure is apparently 
that of homogeneous troostite, and at 700° C the decomposition of 
troostite into sorbite is well advanced. It may be noted on 
referring to the relations between physical properties and micro- 
structure of carbon steels '" that the same general relations exist 
for the high-speed steel quenched from the low-temperature range. 

The micrographs of specimens given the high-temperature 
treatment (Figs. 15 and 16) show a rather astonishing phenome- 
non. Tempered at 200 and 400° C (micrographs a and b) , a well- 
developed, needle-like pattern, suggestive of martensite, is pro- 
duced. When it is considered that the steel is still partially 
austenitic, the propriety of calling this constituent martensite is 
questionable. When tempered at 600° C (micrograph c), where 
secondary hardening first appears, a definite martensitic pattern 
quite similar to that of martensitic carbon tool steel is developed. 
For a tempering temperature of 700° C (micrograph d) , the struc- 
ture is that of the first stages of troostite (of tempering) , and for a 
temperature of 800° C (Fig. 17,0) it is that of sorbite, the structure 
here being practically identical for all quenching temperatures and 
similar to that of the annealed steel. 

The microscopic evidence is positive and confirmatory of the 
physical, namely, that the constituent accompanying the appear- 
ance of secondary hardening is martensite. The nomenclature of 
the patterns developed at 200 and 400° C must, however, await a 
more precise definition of the constituent martensite. 

From the relations pointed out in a previous paper '" between 
the heat evolution (Act) observed on heating hardened carbon 
steels and the accompanying changes in physical properties and 
microstructure, it might be supposed that similar relations exist 
for the high-speed steel. Heating curves were, therefore, taken 
(Fig. 18) of steel A, quenched from three temperatures. The 
inverse-rate method was used, and the thermal characteristics 
noted on the curves are given in Table 2. There is evidence of a 
slight heat evolution (Act) between 600 and 680° C, but it is not 
sufficient^ intense to allow of any positive conclusions. This is 
probably due to the rather limited solubility of the carbide in the 
matrix and the sluggishness of its precipitation, the rapid heating 
rate required by thermal analysis allowing insufficient time for 
its consummation (note the loss in intensity of Ac, with increased 
quenching temperature) . 



534 



Scientific Papers of the Bureau of Standards ivoi. ,6 



'C 

900 



High - 
930X 


Sfreed s/ee/ 
^e£^ from : 
/OSO'C /^OO'C 

! i 


i 
! 
i 

/ 


\ 
) 

\_ 


i ! 

; i 

i 

1 


! 


i 
\ 


j 








1 




Si 


Time 

1 1 i M 


inTerva/ in second 
IS iO /S 

MINI 


s 

1 1 



Fig. i8. — Heating curves of quenched specimens, 
steel A 

TABLE 2. — Transformation Characteristics of Steel A on Heating Following Quenching 



Quenching 
tempera- 
ture 


Rate of 
heating 


Ac, 
maximum 


Ac, 


Beginning 


Maximum End 


-c 

930 
1050 
1200 


° C/sec. 
10 

. 11 
. 11 


"C 

765 
763 
760 


"C 

810 
811 


°C "C 

823 843 
821 844 
815 852 





These curves are in substantial agreement with Carpenter's " 
observations from differential curves on a tungsten-chromium 
high-speed steel, with the exception that he does not recognize 
the transformation Acj, this due to the lack of its characteristic 
features in differential curves. 

" See footnote 3. 



Scott] Heat Treatment of High-Speed Steel 535 

III. SIGNIFICANCE OF THE PHYSICAL CHARACTERISTICS 
OF HIGH-SPEED STEEL 

Inasmuch as the high-grade high-speed steels have much in 
common in spite of the wide range of compositions represented, 
it is permissible to generalize somewhat from the observations 
made on the familiar type of high-speed steel used here. It must 
be remembered, however, that the degree of secondary hardening 
is a function of the carbon and chromium content and also, probably 
to a less degree, of the other alloying elements. 

The rationale of the high-temperattu-e treatment becomes evi- 
dent from an analysis of the physical characteristics. By defining 
red hardness as the resistance to softening by tempering, one may 
see from the hardness versus tempering-temperatitre curves that 
the red hardness increases slightly with quenching temperatiu^e. 
This is evidently one potent reason for a high quenching tempera- 
ture. The initial hardness also increases with the quenching 
temperature, at least in the low-temperature range, and this is a 
further highly desirable characteristic. With the increase in 
hardness there is a corresponding increase in volume and conse- 
quently in volume change on quenching, conditions which favor 
the formation of cracks. If, how^ever, the steel be hardened from 
the high-temperature range with partial austenitization, the density 
change is not increased, and the steel, even if not less hard, is less 
brittle, thus counteracting its tendency to crack and furnishing 
the most satisfactory combination of properties. 

There has been much discussion as to the value ot tempering 
for maximum hardness or secondary hardening. This is evidently 
largely a matter of composition for a given treatment, as this 
factor determines the degree of secondary hardening possible. 
For a steel showing this phenomenon definitely, the increased 
hardness, if not accompanied by greater brittleness, which hardly 
occurs in this case, is certainly of value in a tool, no matter what 
its use. The increase in volume on tempering for secondary 
hardening deserves some consideration on selecting a treatment 
for a given tool. It is apparent that heating in service may change 
the dimensions of an untempered tool to a troublesome degree. 
Tempering for secondary hardening is, therefore, of general advan- 
tage, but it is certainly not of any value to temper at a lower 
temperature where these advantages do not accrue and where 
practically the same degree of effort is expended. 

The comparatively long temperature range, about 100° C, 
in which secondary hardening may be obtained suggests that a 
considerable constitutional difference between the product of the 



536 Scientific Papers of the Bureau of Standards ivoi. i6 

high and the low end of the range may exist, probably correspond- 
ing respectively to martensite and troostite. This being the case, 
it should be of interest to determine whether the low or high 
temperatures giving the same degree of hardness will give the 
better cutting results. 

The sharpness of the changes in the magnetic properties and 
in the density would indicate that these properties furnish a 
valuable test for determining without destruction whether a tool 
has been properly tempered or not. Of the hardness tests, the 
Brinell is of little value on account of the very high degree of 
hardness involved. The scleroscope, however, if properly 
handled, is quite useful, although it is not as sensitive as the 
Brinell to the advancement of the change from martensite to 
troostite. On the whole the physical properties determined for 
a hardened steel, while of course not furnishing a direct criterion 
of its cutting efficiency, do offer a valuable indication of the 
constitution of a given steel, and this is the principal value of 
most physical and mechanical tests. 

IV. SUMMARY 

Attention is called to the importance of fundamental research 
applied to high-speed steel and the value of physical tests for 
this purpose. 

The effect of heat treatment on the density, hardness, micro- 
structure, magnetic properties, and thermal characteristics of 
a standard brand of high-speed steel was determined. The inter- 
pretation of these data permits the following conclusions: 

1. A high-speed steel susceptible to secondary hardening is 
partially austenitic when quenched from a temperature high 
enough to produce this phenomenon. 

2. The microstructtu^e of steels hardened and tempered above 
200° C is similar to that of carbon steels, although the same 
nomenclature in certain cases is not permissible. 

3. The behavior of the physical properties of high-speed steel 
on heat treatment is analogous to that of hypereutectoid carbon 
steel. 

4. The following reasons are given for the use of the high heat 
treatment: (a) Increase of red hardness; (6) increase of initial 
hardness; and (c) reduction of brittleness. 

5. High-speed steel should preferably be tempered for secondary 
hardness. 

Washington, April 27, 1920. 



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